Magnetic Sensing Using T-Matched RAIN RFID Tags

In this paper, we show how wideband measurement of threshold sensitivity and backscatter curves from generic T-matched RAIN (passive UHF) RFID tags can be used for determining magnetic properties of the tagged items. The method is based on measuring all three tag resonances (two for sensitivity and one for backscatter), calculating from those the natural resonant frequency of the tag antenna loop portion and its frequency shift relative to free space, and then extracting effective magnetic permeability. The method is robust to the presence of non-magnetic materials. Possible applications include item sortation for reuse/recycling and smart package inspection.